1. Field of the Invention
The present invention relates to an integrated magneto-optical modulator with optical isolator that integrates an optical isolator used by optical communication and an optical modulator modulating light by utilizing the magneto-optical effect, a process of manufacturing the same, and an optical communication system using the same.
2. Description of the Related Art
An electro-optic modulator such as a Pockels cell has been widely employed in conventional optical communication systems. Particularly, a wave-guide optical modulator utilizing an electro-optic effect of the LiNbO3 crystal is a typical one (Nishihara et al., Optical Integrated Circuit, pp. 298-304, 1985, Ohm-sha). However, the optical modulator using the electro-optic crystal has a disadvantage that it suffers from the DC drift (J. Appl. Phys. Vol. 76 No. 3 pp. 1405-1408, 1994) and optical damage. Therefore, it is difficult to operate it stably for a long time period, or it costs much to avoid deterioration in its characteristics.
On the other hand, although a magneto-optical modulator has been studied for a long time (Appl. Phys. Lett. Vol. 21 No. 8 pp. 394-396, 1972), development thereof is not being well advanced, due to its response speed slower than that of the electro-optic modulator. The conventional magneto-optical modulator employed in an optical communication system disclosed in JP7-199137A responds to no higher than several tens kHz.
Recently, a magneto-optical modulator wherein a DC bias magnetic field is applied to a magneto-optical crystal has been studied in order to measure an electric current in a semiconductor electronic circuit substrate (Appl. Phys. Lett. Vol. 68 No. 25 pp. 3546-3548, 1996, and 61st JJAP Transaction, lecture No. 4p-Q-4, 2000). Furthermore, an optical isolator is used for a magneto-optical modulator (U.S. Pat. No. 6,141,140 or JP3-144417A). Furthermore, both a function of optical modulation and a function of optical isolation can be achieved by using a single magneto-optical element with magnetostatic wave (JP2001-272639A).
However, there are being used, in almost all of the conventional optical communication systems, a direct high speed modulation of the electric current in the semiconductor laser and a wave-guide optical modulator utilizing the electro-optic effect (Pockels effect). Although the direct modulation of the semiconductor laser has an advantage that the optical communication system does not need any other modulators and therefore, its structure becomes simple, the modulation frequency is generally no higher than several GHz. Furthermore, when a semiconductor laser is driven by the high-frequency signal, the drive circuit becomes highly advanced and a transmission distance through the optical fiber of the optical signal is limited by the wavelength chirping.
On the other hand, the electro-optic modulator, particularly the wave-guide optical modulator using the Pockels effect has advantages that it is suitable for a high speed modulation of a laser or LED light and that it is free from a wave-length variation or wave-length chirping which is caused by a direct modulation of a semiconductor laser. However, the electro-optic modulator has a disadvantage that it has a DC drift and optical damage which increase a production cost in order to countermeasure against the disadvantage.
Furthermore, there is a magneto-optical modulator for monitoring an electric current wave form on a micro strip line, by disposing the magneto-optic crystal directly on a semiconductor substrate or micro strip line and by applying a DC bias magnetic field to the magneto-optic crystal (Appl. Phys. Lett. Vol. 68 No. 25 pp. 3546-3548, 1996). However, the above-mentioned current monitoring has a disadvantage that the current wave form is distorted by a ringing due to impedance mismatching between the line and the modulation signal generator, and the above-mentioned current monitoring device (electric current wave monitor) does not include any optical fiber and therefore, is not suitable for the optical communication systems.
On the other hand, another magneto-optic modulator for monitoring the current wave form on the micro strip line wherein an analyzer is disposed after passing a short, e.g., shorter than approximately 1 m, optical fiber (61st JJAP Transaction, Lecture No. 4p-Q-4, 2000). However, a linear polarization becomes in general a random polarization through a long optical fiber. Therefore, the intensity modulation of light propagating through a long optical fiber can not be achieved even by using the analyzer. Further, in the above-mentioned another magneto-optical modulator, the DC bias magnetic field is almost parallel to the high frequency magnetic field. Therefore, the above-mentioned another magneto-optic modulator has a disadvantage that the magneto-optical modulator is magnetically saturated under a large bias magnetic field for obtaining a single magnetic domain and the magnetic saturation greatly reduces or completely extinguishes the modulated signal.
Moreover, there is a significant problem that light returns in the direction toward light source where light propagation must be blocked by the optical isolator when an optical isolator is employed as an optical modulator and the polarization of light rotates with the magneto-optical effect (Faraday effect) being generated by an external magnetic field. In this case, the optical modulator does not perform as an optical isolator at all. Above-mentioned problem will be described in detail, referring to FIGS. 7A, 7B and 8. The incident light propagates through a polarizer 202 from the light source side as shown in FIG. 7A, and only the light, that corresponds to the polarization plane of the polarizer 202, penetrates. And then, the transmitted light through the polarizer 202 is inputted into a magneto-optical element 204 and the plane of polarization rotates by 45° during propagating through it. The analyzer 210 can completely transmit the light to the system side, because the plane of polarization which the analyzer can transmit is equal to the plane of polarization rotated by the polarizer 202. Therefore, the incident light in the forward direction can ideally propagate without loss if the polarization orientation of both of the polarizer 202 and the analyzer 210 is properly set.
On the other hand, when light in the reverse direction is introduced from the system side, the analyzer 210 transmits the light with just the same plane of polarization as the polarization orientation of the analyzer 210. And then, the transmitted light through the analyzer 210 is inputted into a magneto-optical element 204 and the plane of polarization rotates by 45° during propagating through it. The rotation direction of the polarization is always the same regardless of the forward or reverse direction. As the polarization of the penetrating light through the magneto-optical element 204 is absolutely perpendicular to the polarization orientation of the analyzer 202, the reflected feedback light can not return toward the light source at all. This structure can be called an optical diode because light can propagate in the one-way direction (the forward direction). However, above-mentioned operation will be achieved when the rotation angle of the polarization due to Faraday effect at the magneto-optical element 204 is strictly 45°. That means, if the rotation angle thereof slightly shifts from 45°, the polarization of the reflected feedback light through the magneto-optical element 204 is not absolutely perpendicular to the plane of polarization at the analyzer 202, and the reflected feedback light slightly returns toward the light source. In the particular case of using an optical modulator as an optical isolator, the Faraday rotation angle at the magneto-optical element 204 needs to shift from 45° because the magneto-optical element 204 must execute the light modulation. The greater the shift of angle is, the more the reflected feedback light returns toward the light source.
FIG. 8 shows a graph of the relationship between modulation depth and transmittance of reflected feedback light when an optical isolator is used as an optical modulator. An usual optical isolator should function so that light transmittance in the reverse direction may become 0.1% or less (0.001% or less according to the usage). However, it has been known that the reflected feedback light returns to the light source by approximately as much as 10%-20% even if modulation depth becomes a few percentages as shown in FIG. 8, and the optical isolator does not carry out the function of optical isolation at all. So far, it was not considered at all that the function as the optical isolator was remarkably deteriorated like this when the optical isolator was employed as the optical modulator. In addition, an optical isolator may be covered with a metal package, and the magnet of the rare earth metal may be used in an optical isolator. Therefore, there is also the problem that the high-frequency magnetic field for modulation can not be effectively applied to the magneto-optical element by the influence of the eddy current when the high-frequency magnetic field is applied from the exterior of the optical isolator.
Furthermore, there is a problem when the magneto-optical modulator using the magnetostatic wave and the optical isolator are composed of a single element (JP2001-272639A), a broadband communication such as an optical communication can not be achieved because the magnetostatic wave is excited only by a narrow frequency bandwidth. In the same case, there is also another problem that the optical isolator can not efficiently block the reflected feedback light and the reflected feedback light returns more toward the light source when the depth of modulation in the optical modulator becomes greater.